Method and apparatus for use with a twin-wire press

ABSTRACT

A method for dewatering a fiber suspension in a twin-wire press is disclosed including feeding the fiber suspension to an inlet end of an elongated dewatering space, dewatering the fiber suspension during displacement of the endless lower and upper wires which form the elongated dewatering space by flowing filtrate through the endless lower and upper wires, collecting the filtrate in outlet boxes, and adjusting the pressure differential between the elongated dewatering space and each of the upper and lower outlet boxes by controlling the counter-pressure applied to the filtrate flow on both sides of the endless upper and lower wires directly adjacent the inlet end of the dewatering space. A twin-wire press is also disclosed therein.

FIELD OF THE INVENTION

The present invention relates to a method and device for use with atwin-wire press.

BACKGROUND OF THE INVENTION

Twin-wire presses for dewatering of fiber suspensions and forming acontinuous web thereof are previously known. Dewatering of the pulp isusually from an inlet pulp concentration of from 3-8 percent by weightto outlet pulp concentrations of 30-50 percent by weight. According tothe state of the art, such twin-wire presses comprises lower rolls, anendless lower wire running in a path around the lower rolls, upperrolls, and an endless upper wire running in a path around the upperrolls. The two wires co-operate with each other along sections of thesepaths that run substantially in parallel with each other for dewateringof the fiber suspension between the wires during displacement thereof.An inlet box provides for supply of the fiber suspension to awedge-shaped dewatering space between the wires. The twin-wire pressfurther comprises two dewatering tables supporting the respective wirein these sections of the path and forming the wedge-shaped dewateringspace between the wires for initially pressing and dewatering the fibersuspension, whereby a web is formed between the wires, and a rollarrangement situated after the dewatering tables in those sections ofthe paths, as seen in the direction of movement of the wires, forfinally pressing and dewatering the web between the wires, so that theweb will obtain a desired dryness. By dewatering space is meant thesection between the dewatering tables where dewatering occurs. Alongsidethe longitudinal direction of the wires, in the wedge-shaped dewateringspace, there are perforated dewatering elements that are arrangedagainst the wires outside the dewatering space, through which dewateredfiltrate is lead away to upper and lower outlet boxes, respectively,arranged at the dewatering tables for receiving filtrate that flows fromthe dewatering space through the upper and lower wires, respectively.The dewatering elements which rest against the upper and lower wires,respectively, in the wedge zone constitute together an upper and lowerdewatering surface, respectively, where each dewatering surface may becomposed of one or more dewatering elements. Upper and lower outletboxes may be divided into several chambers whereby a filtrate throughthe upper and lower dewatering surfaces, respectively, may be dividedinto partial filtrate in two or more chambers in respective outletboxes.

A traditional dewatering space in a twin-wire press has a wedge-shapewith a fixed design that is not changeable when the twin-wire press isin operation. The outlet boxes are not sealed, and thus work againstatmospheric pressure. The geometry of the table and the pulp suspensionflow creates the operating pressure difference over the wire thatcontrols the dewatering. The wedge shape determines the pressurebuild-up in the twin-wire press, and the dewatering process is to alarge extent dependant on the shape of wedge, which is difficult tochange. Changes of the wedge-shape requires new, extensive settings ofthe dewatering tables, a change of side sealings to the dewateringtables, etc. Owing to the fact that the wedge-shaped dewatering spacehas its largest cross-sectional area adjacent the inlet box, and thusnarrows linearly in the direction of movement of the web, a well adaptedpressure difference, also called delta P, is not obtained over the wirein the area at the dewatering space inlet end, which results in adefective build-up of the fiber web. Small fibers accumulate closest tothe wire surface, and create a layer which is difficult for the filtrateto penetrate. The problem is to create a geometry for the dewateringtable that provides an optimal pressure difference for the mosteffective dewatering and formation of the fiber web. The dewateringfirst begins some distance inside the dewatering space in the positionwhere a first dewatering element is located. The inlet box comprisesextended sealing blades whose free ends are arranged against the insidesof the wires in the dewatering space. The ends of the sealing bladesterminate at a position usually situated about 100 mm in advance of thelocation of the first dewatering element, which results in a relativelyextensive backward flowing leakage of thick, fiber-rich flow occurs onboth sides of the wires, where the ends of the sealing blades terminate.

One of the objects of the present invention is to provide an easier andimproved adjustment of the pressure levels, and thereby more favorabledewatering, over the whole length of the dewatering table in a twin-wirepress without changing the geometry of the dewatering table. Anotherobject is to minimize leakage of fibers, particularly at the inlet endof the dewatering space.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other objects havenow been realized by the invention of a method for dewatering a fibersuspension in a twin-wire press, the method comprising feeding the fibersuspension to an inlet end of an elongated dewatering space defined byan endless lower wire and an endless upper wire, dewatering the fibersuspension in the elongated dewatering space during displacement of theendless lower and upper wires by forming a flow of filtrate from theelongated dewatering space through the endless lower and upper wires,collecting the filtrate in a pair of upper and lower outlet boxes, andadjusting the pressure differential between the elongated dewateringspace and each of the pair of upper and lower outlet boxes bycontrolling the counter-pressure applied to the flow of filtrate on bothsides of the endless upper and lower wires directly adjacent to theinlet end of the elongated dewatering space. Preferably, the methodincludes applying a counter-pressure to the flow of the filtrate at aplurality of separate locations along the elongated dewatering space. Ina preferred embodiment, the method includes independently controllingthe counter-pressure at the plurality of separate locations along theelongated dewatering space.

In accordance with the present invention, a twin-wire press has alsobeen devised for accomplishing these objects. The twin-wire press fordewatering a fibrous suspension comprises an endless lower wire and anendless upper wire forming an elongated dewatering space including aninlet and for dewatering the fibrous suspension therebetween, an inletbox for applying the fiber suspension to the inlet end of the elongateddewatering space, a lower perforated dewatering element disposedadjacent to the lower wire on the outer surface thereof, an upperperforated dewatering element disposed adjacent to the upper wire on theupper surface thereof, a lower outlet box for receiving the filtrateflowing from the elongated dewatering space through the elongated lowerwire, an upper outlet box for receiving the filtrate flowing from theelongated dewatering space through the elongated upper wire, and apressure control device for controlling the counter-pressure in theupper and lower outlet boxes in order to adjust the pressuredifferential between the elongated dewatering space and both the upperand lower outlet boxes at the inlet end of the elongated dewateringspace. In a preferred embodiment of the twin-wire press, the pressurecontrol device comprises a filtrate outlet pipe for receiving the flowof the filtrate from the upper and lower outlet boxes and a spillwayoverflow disposed at a level above the elongated dewatering space in thefiltrate outlet pipe. Preferably, the level of the spillway overflow isadjustable.

In accordance with one embodiment of the twin-wire press of the presentinvention, each of the upper and lower outlet boxes comprises aplurality of chambers for receiving separate portions of the filtrate.In a preferred embodiment, the twin-wire press includes a plurality ofpressure control devices for independently controlling thecounter-pressures for each of the plurality of the chambers. In anotherembodiment, the twin-wire press includes sealing blades extending fromthe inlet box directly adjacent to the upper and lower perforateddewatering elements including free ends arranged against the inside ofthe endless upper and lower wires whereby a first filtrate portion isremoved from the elongated dewatering space to a one of the plurality ofchambers of both the upper and lower outlet boxes.

The above objects are also achieved by a method for dewatering a fibersuspension in a twin-wire press according to the present invention, inwhich method the fibre suspension is fed to an oblong dewatering spacedefined by an endless lower wire and an endless upper wire of thetwin-wire press, against which wires lower and upper perforateddewatering elements are arranged outside the dewatering space, and thefiber suspension is dewatered in the dewatering space duringdisplacement of the wires, such that a flow of filtrate from thedewatering space through the wires and the perforated dewateringelements is formed, filtrate that flows from the dewatering spacethrough the wires is collected in outlet boxes, and the pressuredifference between the dewatering space and at least one outlet box isadjusted by controlling at least one counter-pressure that is applied onat least a part of the flow of filtrate.

By means of control of the pressure in the dewatering space inaccordance with the present invention, the dewatering can be correctedwithout needing to change the geometry of the wedge-shaped dewateringspace, desired basis weight of the fiber web can be maintained, and newsettings of the dewatering tables need not be carried out. The presentinvention facilitates different pressures being obtained at differentareas along the whole dewatering space, whereby a perfect pressure forthe dewatering required can be achieved.

The build-up of the fiber web is improved, and the fiber web can beprepared in a desired way before the roll nips. The present inventionmakes it possible to decrease the pressure difference in a first sectionof the dewatering space such that the smaller fibers do not accumulateclosest to the surface of the wire.

According to the present invention, the fiber suspension enters into thedewatering space through an inlet end. Preferably, the pressuredifference is adjusted between the dewatering space and outlet boxes bycontrol of at least a counter-pressure that is applied to at least aportion of the flow of filtrate. Suitably at least a counter-pressure isapplied to the portion of the flow of filtrate that is formed on bothsides of the lower and upper wires directly adjacent to the inlet end ofthe dewatering space, particularly in the area around the sealingblades. By inlet end of the dewatering space is meant the position wherethe dewatering space begins. The possibility of creating a firstdewatering area with separate pressure control for separation of a firstfiltrate is particularly advantageous, because this first filtrateconsists of a relatively thick, fiber-rich flow because the built-up ofthe fiber web does not begin until some distance into the dewateringspace. Accordingly, fiber leakage at the dewatering inlet end can beminimized.

According to a preferred embodiment of the present invention it ispossible to apply a plurality of separate counter-pressures onrespective portions of the flow of filtrate, which are separated,whereby the pressure level over the whole length of the dewatering spacecan be optimally adjusted. Thus, the fiber web can be better preparedand controlled before the nips. The separate counter-pressures cansuitably be controlled independently of each other for an optimaladjustment.

The present invention also relates to a twin-wire press for dewateringof a fiber suspension, comprising an endless lower wire and an endlessupper wire, which defines an oblong dewatering space in which the fibersuspension will be dewatered during displacement of the wires, an inletbox for supplying the fiber suspension to the dewatering space, lowerand upper perforated dewatering elements that are arranged against wiresoutside the dewatering space, a lower outlet box for receiving filtratethat flows from the dewatering space through the lower wire and lowerperforated dewatering element, an upper outlet box for receivingfiltrate that flows from the dewatering space through the upper wire andupper perforated dewatering element, and a pressure control devicearranged to control one or more counter-pressures in the lower and upperoutlet box, respectively, for adjustment of a pressure differencebetween the dewatering space and the outlet boxes.

The outlet boxes can be separated from each other whereby a pressurecontrol device is arranged for each of the outlet boxes in order to beable to receive different counter-pressures in the outlet boxes. Thepressure control device may, for example, be provided with a pump and avalve or the like, which facilitates that control of the pressuredifference can be achieved between the dewatering space and the outletboxes. According to a preferred embodiment of the present invention, thepressure control device comprises a filtrate outlet pipe connected tothe outlet boxes, and a spillway overflow in the filtrate outlet pipe,whereby the spillway overflow is located at a level above the dewateringspace. The level of the spillway overflow is suitably adjustable, andthus its height over the dewatering space can be changed, which meansthat the pressure difference can be adjusted optimally between thedewatering space and the outlet boxes.

As described above, according to a preferred embodiment of the presentinvention, at least a counter-pressure can be applied on both sides ofthe upper and lower wires directly adjacent to the inlet end of thedewatering space, in particular in the area around the sealing blades.In that respect, the twin-wire press comprises initial perforateddewatering elements arranged directly adjacent to sealing blades, whichextend from the inlet box, whose free ends are arranged against theinsides of the wire in the dewatering space, whereby a first partialfiltrate is separated to a first chamber of a respective outlet box.

One or more upper and lower perforated dewatering elements forms anupper and lower dewatering surface, respectively. According to anembodiment of the present invention, the outlet boxes of the twin-wirepress comprise two or more chambers for receiving a respective partialfiltrate, which facilitates that the pressure level over the wholelength of the dewatering space can be optimally controlled. Preferably,a plurality of separate pressure control devices can be arranged tocontrol the counter-pressures in the chambers independently of eachother.

The chamber, or those chambers that are arranged to use acounter-pressure, are sealed in order to operate above atmosphericpressure. In the case where the outlet boxes are divided into two ormore chambers, the first chamber is the one that is arranged to receivefiltrate from the first section of the dewatering space.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail in thefollowing detailed description, with reference to accompanying drawings,without restricted interpretation of the invention thereof, where

FIG. 1 is a side, elevational, schematic, partial cross-sectional viewthrough a twin-wire press according to the present invention, and

FIG. 2 is a side, elevational, schematic view through the entiretwin-wire press according to FIG. 1.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 shows schematically a partial viewthrough the twin-wire press 2 according to the present invention, moreprecisely a section at an inlet end 14 of a dewatering space 4 in thearea at and around an inlet box 16. The fiber suspension that will bedewatered is supplied from the inlet box 16 to the dewatering space 4defined by an endless lower wire 6 and an endless upper wire 8 duringdisplacement of the wires, 6 and 8. Initial perforated dewateringelements, 10 and 12, are arranged outside the dewatering space. Suchperforated dewatering elements are used over the entire length of thedewatering space for removal of filtrate from the dewatering space. Theperforated dewatering elements, 10 and 12, shown in FIG. 1 are arrangedat the inlet end 14, and are formed of a bent plate with holes therein,which replaces those rolls that are traditionally used at an inlet endof a twin-wire press. Filtrate flows through the wires, 6 and 8, fromthe dewatering space and is collected in outlet boxes, 18 and 20. Thelower outlet box 18 receives filtrate that flows from the dewateringspace through the lower wire 6 and lower dewatering element while theupper outlet box 20 receives filtrate that flows from the dewateringspace through the upper wire 8 and upper dewatering element. A pressurecontrol device 22 controls a counter-pressure in the lower and the upperoutlet boxes, 18 and 20, respectively, and is arranged to adjust thepressure difference between the dewatering space and the outlet boxes,18 and 20.

FIG. 1 shows a pressure control device 22 that comprises a verticalfiltrate outlet pipe 24 connected to the outlet boxes, and a spillwayoverflow 26 in the filtrate outlet pipe, whereby the spillway overflowis located at a level 28 above the dewatering space 4. The level of thespillway overflow is thus the difference in height between the spillwayoverflow and the dewatering space. The filtrate outlet pipe 24 isarranged to be vertically displaceable, whereby the level 28 of thespillway overflow 26 relative to the dewatering space, and the outletboxes therein, are adjustable. Thus, the pressure difference between thedewatering space 4 and the outlet boxes, 18 and 20, may easily becontrolled by displacement of the filtrate outlet pipe 24 whereby theposition of the spillway overflow 26 can be moved. In that respect, thefiltrate outlet pipe 24 can, for example, be formed of a plurality ofpipes telescopically arranged with respect to each other.

As is evident from FIG. 1, the present invention according to apreferred embodiment can comprise initial perforated dewateringelements, 10 and 12, arranged directly adjacent to sealing blades, 34and 36, extended from the inlet box 16, whose free ends are arrangedagainst an inside, 38 and 40, respectively, of the wires, 6 and 8, inthe dewatering space, whereby a first filtrate is removed to a firstchamber, 30 and 32, of the outlet boxes, 18 and 20, respectively.

Thus, each outlet box, 18 and 20, may comprise more than one chamber forreceiving a partial filtrate, respectively, from the dewatering space 4,and the pressure control device 22 can be arranged to control thecounter-pressures in the chambers independently of each other. Thus, thepressure control device 22 may, for example, be arranged to control thecounter-pressure in the chambers, 30 and 32, independently of eachother.

During operation of the twin-wire press 2 according to FIG. 1,displacement of the wires, 6 and 8, occurs during rotation of the rolls(not shown). The fiber suspension that will be dewatered is fed to anoblong dewatering space 4 through an inlet end 14 thereof. The fibersuspension is dewatered in the dewatering space during displacement ofthe wires, such that a flow of filtrate from the dewatering space 4through the wires, 6 and 8, and the perforated dewatering elements, 10and 12, is formed.

Filtrate that flows from the dewatering space through the wires iscollected in the separate chambers, 30 and 32, of the outlet boxes, 18and 20. The pressure difference between the dewatering space 4 and thechambers, 30 and 32, is adjusted by applying at least a counter-pressureon the portion of the flow of filtrate that is formed on both sides ofthe upper and lower wires, 6 and 8, directly adjacent to the inlet end14 of the dewatering space. The magnitude of the counter-pressure isadjusted by the location of the level 28 of the spillway overflow 26. Inthat respect, a relatively thick, fiber-rich first filtrate can beremoved at the inlet end 14 and the build-up of the fiber web, duringfavourable conditions, can begin almost immediately after feeding intothe dewatering space 4.

FIG. 2 shows the entire twin-wire press 2 in FIG. 1 according to thepresent invention. The twin-wire press 2 comprises three lower rolls,more specifically a drive roll 50, a guide roll 52 and a stretch roll54. The above mentioned (FIG. 1) endless lower wire 6 runs in a patharound the lower rolls, 50, 52, and 54. In a corresponding way the abovementioned (FIG. 1) upper endless upper wire 8 runs in a path aroundthree upper rolls, specifically a drive roll 56, a guide roll 58 and astretch roll 60. An upper dewatering table 62 that supports the upperwire 8, and a lower dewatering table 64 that supports the lower wire 6,forms the dewatering space 4 between the wires, 6 and 8. FIG. 2 alsoshows the pressure control device 22, that has been described above withreference to FIG. 1. “Press section” in FIG. 2 indicates an ordinaryroll arrangement according to the state of the art.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method for dewatering a fiber suspension in a twin-wire press, saidmethod comprising feeding said fiber suspension to an inlet end of anelongated dewatering space defined by an endless lower wire and anendless upper wire, dewatering said fiber suspension in said elongateddewatering space during displacement of said endless lower and upperwires by forming a flow of filtrate from said elongated dewatering spacethrough said endless lower and upper wires, collecting said filtrate ina pair of upper and lower outlet boxes, and adjusting the pressuredifferential between said elongated dewatering space and each of saidpair of upper and lower outlet boxes by controlling the counter-pressureapplied to said flow of filtrate on both sides of said endless upper andlower wires directly adjacent to said inlet end of said elongateddewatering space.
 2. The method of claim 1 including applying acounter-pressure to said flow of said filtrate at a plurality ofseparate locations along said elongated dewatering space.
 3. The methodof claim 2 including independently controlling said counter-pressure atsaid plurality of separate locations along said elongated dewateringspace.
 4. A twin-wire press for dewatering a fibrous suspensioncomprising an endless lower wire and an endless upper wire forming anelongated dewatering space including an inlet and for dewatering saidfibrous suspension therebetween, an inlet box for applying said fibersuspension to said inlet end of said elongated dewatering space, a lowerperforated dewatering element disposed adjacent to said lower wire onthe outer surface thereof, an upper perforated dewatering elementdisposed adjacent to said upper wire on the upper surface thereof, alower outlet box for receiving said filtrate flowing from said elongateddewatering space through said elongated lower wire, an upper outlet boxfor receiving said filtrate flowing from said elongated dewatering spacethrough said elongated upper wire, and a pressure control device forcontrolling the counter-pressure in said upper and lower outlet boxes inorder to adjust the pressure differential between said elongateddewatering space and both said upper and lower outlet boxes at saidinlet end of said elongated dewatering space.
 5. The twin-wire press ofclaim 4 wherein said pressure control device comprises a filtrate outletpipe for receiving the flow of said filtrate from said upper and loweroutlet boxes and a spillway overflow disposed at a level above saidelongated dewatering space in said filtrate outlet pipe.
 6. Thetwin-wire press of claim 5 wherein said level of said spillway overflowis adjustable.
 7. The twin-wire press of claim 4 wherein each of saidupper and lower outlet boxes comprises a plurality of chambers forreceiving separate portions of said filtrate.
 8. The twin-wire press ofclaim 7 including a plurality of pressure control devices forindependently controlling said counter-pressures for each of saidplurality of chambers.
 9. The twin-wire press of claim 7 includingsealing blades extending from said inlet box directly adjacent to saidupper and lower perforated dewatering elements including free endsarranged against the inside of said endless upper and lower wireswhereby a first filtrate portion is removed from said elongateddewatering space to one of said plurality of chambers of both said upperand lower outlet boxes.